1. Field of the Invention
The present invention relates to an improvement of a distance measuring apparatus whereby distance of a photographic object can be measured accurately irrespective of its visual angle from the apparatus.
2. Prior Art
A distance measuring apparatus which projects light of a considerably wide solid angle towards an object, receives reflected light by a photoelectric device having a narrow directivity and outputs a distance signal as a function signal of the output signal of the photoelectric device which output signal varies responding to distance to the object, is known by the patent specification of the U.S. Pat. No. 1,866,581.
In the above-mentioned distance measuring apparatus, in order to avoid inaccurate measurement it is necessary to select the directivity of the photoelectric device sufficiently narrow with respect to the visual angle of the measured object from the photoelectric device. The reason of the necessity of the narrow directivity of the photoelectric device is explained referring to FIG. 1, wherein a light source 1 projects light of a considerably wide solid angle, and a photoelectric device 2 such as a phototransistor of a very narrow directivity receives the light reflected by the object 3. Now provided that the light source 1 projects light of the flux density L per unit solid angle and the photoelectric device 2 has a directivity of solid angle .alpha., and its optical axis is disposed towards the center of the object 3 of a disc of an area S and of a reflection index m, then, when the object 3 is disposed at the distance of ##EQU1## the object 3 makes a solid angle which is equal to that of the directivity angle .alpha. of the photoelectric element 2. In other words, the area S of the object is equal to such area at the distance R that reflected light from which area induces output signal to the photoelectric device 2.
(i) For the distance R in a shorter range of ##EQU2## the intensity of light Q received by the photoelectric device 2 is proportional to the illumination mL/R.sup.2 of the object. By considering that the light flux density L is constant for a short time considered, the following formula holds: ##EQU3## In the above-mentioned system, when the electric circuit including the photoelectric device is designed to issue an output signal Y of the value Y.alpha.k.sqroot.1/Q, then the following equation holds: ##EQU4##
In the actual system, as is shown in the equation (3), the output signal Y is proportional to the distance R and inversely proportional to the square root of the reflection index m. Since the output signal Y is inversely proportional to the square root of m, variation of m only slightly affects the output value Y. For example, when k is selected as k=.sqroot.0.4, the output Y changes as follows responding to three values of m: ##EQU5##
When the actual distance is 1 m, then the above-mentioned three outputs make a distance meter indicate in a range of 0.7-1.4 m, and for a practical distance meter for usual photographing, such variation is permitted.
(ii) For the distance R in a longer range of ##EQU6## the intensity of light received by the photoelectric device 2 is proportional to the illumination m/R.sup.2, and at the same time to S/.alpha.R.sup.2, which is the ratio of area of the object 3 to such area at the distance R that reflected light from which area induces output signal to the photoelectric device 2. Accordingly, for the intensity of light Q, the following formula holds: ##EQU7##
Then, when this light input is converted into an electric signal Y by the same circuit as the foregoing case, which is designed to issue an output signal Y of the value Y.alpha.k.sqroot.1/Q, then the following equation holds: ##EQU8## In this case, when k is selected as k=.sqroot.0.4, the output Y changes as follows responding to the values of m: ##EQU9## This means that, on top of the variation range of 0.7R to 1.4R in the foregoing case (i), a further factor .sqroot..alpha.R.sup.2 /S is added. The value of the factor .sqroot..alpha.R.sup.2 /S at the distance R is about 1.6, and therefore, the output signal varies as follows responding to three values of m:
______________________________________ when m = 0.2, Y = 0.7 .times. 1.6R = 1.2R when m = 0.4, Y = 1.0 .times. 1.6R = 1.6R and when m = 0.8, Y = 1.4 .times. 1.6R = 2.2R. ______________________________________
When the actual distance is 1 m, the indication of the measured distance ranges from 1.12 m to 2.2 m, and the latter indication is not tolerable any more.
As has been explained, for the distance R of the range ##EQU10## the indication of the distance may become much different from the actual distance. In other words, in order to avoid such discrepancy, the condition of the formula R &lt;.sqroot.S/.alpha. must be fulfilled by, for example, selecting the solid angle .alpha. narrow. Hereupon, another trouble arises when the solid angle .alpha. of the directivity of photoelectric device is selected narrow. The handling of the distance measuring apparatus becomes very difficult since the optical axis of the photoelectric device must be very accurately directed towards the object, and use of the apparatus for an easy-handling camera becomes impractical.
Another problem of the distance measuring apparatus is the undesirable effect of background noise of sunlight and illuminating light to the photoelectric device, and the effect of such background noise should be eliminated as much as possible.